Adaptive time slot scheduling apparatus and method for end-points in an ATM network

ABSTRACT

A scheduling mechanism for satisfying specified Quality of Service guarantees for each VC of an end-point host in an ATM network and for satisfying low latency requirements of certain VCs while maintaining high throughput. The scheduling mechanism includes at least one time slot ring, a VC table and at least two pending queues. The time slot ring is an array of time slots grouped into clusters. Each slot contains a pointer which contains the virtual channel identifier to be serviced. The VC table is an array of all the VC descriptors. The pending queues are used for queuing a new VC and when the scheduler is unable to find a time slot for a VC it is queued in the pending queue. One pending queue is restricted to VCs requiring low latency and thus need to be serviced prior to servicing other VCs not sharing the same low latency requirements. A second pending queue is used for VCs with a less critical latency requirement. The ring contains cluster pointers which point to the first VC to be serviced. The rest of the VCs in the cluster are linked to the first VC. In order to accommodate different classes of service, each entry in the ring holds multiple cluster pointers. The scheduler first processes all low latency VCs, and then circularly reads and processes one cluster at a time at the maximum speed allowed by the physical link, and a current slot pointer points to the slot being serviced. For the current time slot, the scheduler first services the VC waiting on this slot, then schedules a new target time slot for this VC after marking the current slot empty.

CROSS REFERENCE TO RELATED APPLICATIONS

The present patent application is a divisional of U.S. patentapplication Ser. No. 08/758,611, entitled ADAPTIVE TIME SLOT SCHEDULINGAPPARATUS AND METHOD FOR END-POINTS IN AN ATM NETWORK, having a filingdate of Nov. 27, 1996 now U.S. Pat. No. 5,914,934, which is acontinuation-in-part of U.S. patent application Ser. No. 08/580,470,entitled ADAPTIVE TIME SLOT SCHEDULING APPARATUS AND METHOD FOREND-POINTS IN AN ATM NETWORK, that application having a filing date ofDec. 28, 1995, which is now U.S. Pat. No. 5,751,709, and which isrelated to U.S. patent application Ser. No. 08/579,961, entitledADAPTIVE TIME SLOT SCHEDULING APPARATUS AND METHOD UTILIZING A LINKEDLIST MECHANISM, having a filing date of Dec. 28, 1995, which is now U.S.Pat. No. 5,712,851. Present, parent, and related applications andpatents have a common assignee, one or more common inventors, and arecopending. The parent application and related U.S. Patents are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field oftelecommunications systems, and more particularly to an adaptive timeslot scheduling scheme for end-points in ATM networks and particularlysuited for providing low latency to a lock manager function indistributed processing systems.

BACKGROUND OF THE INVENTION

Asynchronous Transfer Mode (ATM) networks allow a large number of dataflow paths or Virtual Channels (VCs) to be statistically multiplexedover a common link. A high speed ATM connection allows a host end-pointdevice, for example, a computer, to have a large number of simultaneousvirtual channels sharing bandwidth of the common link. A challenge inimplementing the large number of virtual channels is to achievespecified Quality of Service (QOS) for each virtual channel independentof other virtual channels. The challenge is compounded when a mix ofvirtual channels with differing requirements for the QOS have to besatisfied.

In order to efficiently accommodate the differing QOS requirements ofmultiple virtual channels, a scheduler in an end-point device should beable to respond quickly to changes in transmission rates on theindividual virtual channels. This is required, for example, for theVariable Bit Rate (VBR) and the best effort or Available Bit Rate (ABR)classes of service. The scheduler should also be able to dynamicallyadjust the scheduling when new virtual channels are added or old onesare removed or existing virtual channels stop or start sending datatemporarily. In addition, the scheduler should also minimize the jitter,that is, changes in cell rate. This is important because ATM switches,which receive streams of cells generated by the scheduler, continuouslymonitor each VC using a leaky bucket algorithm or Generic Cell RateAlgorithm (GCRA) to check if the switch is adhering to the trafficcontract. In the event the leaky bucket overflows, the cells canpotentially be dropped by the switch. Accordingly, the scheduler shouldprotect against this by minimizing the jitter. In case of constant bitrate (CBR) channels, a buffer is required at the receiving end to removethe jitter and smooth the incoming flow of cells. Increased jitter willthen require a larger buffer.

Scheduling schemes have been proposed in the prior art for schedulingVCs within a switch and within an end-point. Heretofore, these schemeshave been unable to meet the requirements necessary for efficientimplementation of multiple virtual channels having various transmissionrates and differing QOS guarantees. Accordingly, there is a need for ascheduling mechanism capable of implementing specified transmissionrates for each virtual channel independent of other virtual channels,wherein the virtual channels include a mix of differing QOSrequirements. U.S. patent application Ser. No. 08/580,470, the parent ofthe present application addresses this need.

A scheduler in an end-point device should also be able to accommodate alow latency QOS requirement of an individual virtual channel. Oneapplication where low latency is of concern is the use of an ATM networkto build a parallel processing system where individual workstations actas nodes and the ATM network functions as the fabric for inter-processorcommunication. In this ATM network application it is necessary to have alock manager, whose messages are typically short and infrequent, forlocking a resource when it is accessed by one processor, therebypreventing access to the resource to more than one processor at a time.Since a principal objective for using the ATM network for parallelprocessing is the potential for increased throughput, the ATM network isa stable fabric. However, the increased throughput is achieved at theexpense of high latency. With respect to the lock manager, this cost isvery serious as end points are prevented from accessing a resource untilit is released by the lock manager operation. Thus reducing latency forthe lock manager is critical to realizing high throughout.

SUMMARY OF THE INVENTION

The present invention discloses an apparatus and method for realizinglow latency while maintaining high throughput for high priorityoperations such as lock manager, conveyed in a specific virtual channel(VC) generated by an end-point host in an ATM network. In order tosatisfy the specified low latency requirement for the specific VC, thepresent invention uses a scheduler which utilizes a time slot ring. Asthe VC's with the less critical latency requirements have specific ratesin terms of cells per second, the time slot ring is only defined forthese VC's. The scheduler dynamically fills in the various slots of thetime slot ring with different VCs to be serviced, and services them onlyafter first servicing all VCs having a low latency requirement.

In one preferred embodiment of the present invention, the schedulerincludes a time slot ring, a VC table and at least two pending queues.The time slot ring is an array, wherein each element represents a timeslot. The size of the time slot ring is determined by the ratio of thehighest rate to the lowest rate to be supported. The VC table is anarray of all the VC descriptors. The pending queues are used for queuinga new VC. At least two pending queues are used; one for those VCsrequiring low latency and therefore need to be serviced first and theother or others for VCs with less strict latency requirements so thatthey can be serviced after the first pending queue is empty. Where morethan two pending queues are used, service of various levels of latencyis possible.

The size of the ring and consequently the memory required to store thering can be reduced by combining a fixed number of consecutive timeslots. Each cluster nominally holds a “Cluster-Size” number of slots.The ring contains cluster pointers which point to the first VC to beserviced. The rest of the VCs in the cluster are linked to the first VC.In order to accommodate different classes of service, each entry in thering holds two cluster pointers: the High Priority Pointer (HPP) whichservices the VBR/CBR traffic classes and a Low Priority Pointer (LPP)which services VCs belonging to the ABR/VBR classes.

In calculating the target slot, the scheduler calculates the targetcluster. This acts to decrease the memory requirements, but at theexpense of reduced accuracy in calculating the target slot since it hasto be approximated to the nearest cluster. A variable called the CCTP(Current Cell Time Pointer) is used in all the timing calculations. TheCCTP increments every time slot without regard to the cluster size andensures accuracy of time delay calculations. A Current Cell Time Pointer(CCTP) increments every cell time and points to the cell position whichwill be sent if the scheduler is not backlogged. A set of Current SlotPointers (CSHPP and CSLPP) point to the current slot which is beingserviced. In a second embodiment of the present invention, a pluralityof time slot rings are used to accommodate multiple classes of trafficas defined by the ATM Forum Traffic Management Specification 4.0, herebyincorporated by reference as if fully set forth herein. Multiple timeslot rings allow different traffic classes to have different prioritiesdependent on how the scheduler services the time slot rings. One exampleof this embodiment uses two rings: a high priority ring for VBR/CBRtraffic, and a low priority ring for ABR/VBR traffic.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present invention, reference may behad to the following description of exemplary embodiments thereof,considered in conjunction with the accompanying drawings, in which:

FIG. 1 shows a diagram of basic elements included in the presentinvention adaptive time slot scheduling mechanism;

FIG. 2 shows an alternate implementation of a time slot ring accordingto the present invention; and

FIG. 3 shows an alternate two ring embodiment of the present inventionwhere high bit rate VCs are serviced by a fine bin size ring and low bitrate VCs are services by coarse bin size rings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses an apparatus and method for adaptivetime slot scheduling at end-points in an ATM network which provides lowlatency to critical operations while maintaining high throughput.Referring to FIG. 1, there is shown a block diagram illustrating onepreferred embodiment of a scheduling mechanism 10 according to thepresent invention. As shown, the scheduling mechanism or scheduler 10 iscontained, for example, in an end-point host of an ATM network. Thebasic components of scheduler 10 include a time slot ring 12, a VC table14 and at least two pending queues 16 and 17. The time slot ring 12 isessentially an array found in memory, for example, wherein each elementin the array represents a specific time slot 18. Each time slot 18contains a pointer which includes the virtual channel identifier (VCID)which identifies the virtual channel to be serviced in this time slot.The VCID points to a VC descriptor 20 in the VC table 14 which storesall the information regarding the specific VC. If the slot 18 is empty,a null pointer is contained therein.

Pending queues 16 and 17 are queues of VCIDs and used for queuing a newVC. As shown in FIG. 1, pending queue 17 is a queue of VCIDs having aquality of service requirement of low latency, and pending queue 16 is aqueue of all VCIDs other than those in pending queue 17. In accordancewith the present invention, pending queue 17 has priority over pendingqueue 16 at the time of scheduling. In a preferred embodiment of thepresent invention, the scheduler is included as part of a Segmentationand Reassembly (SAR) in an Application Specific Integrated Circuit(ASIC).

In the basic operation of the scheduler 10 shown in FIG. 1, thescheduler 10 in conjunction with a built-in processor reads andprocesses one slot at a time in the time slot ring at a predeterminedrate, for example, the maximum speed that the physical link of theend-point device will allow. The scheduler 10 then circularly servicesthe slots in ring 12 in a continuous fashion. A current slot pointer(CSP) 22 points to the slot being serviced. For a current time slot,i.e., the slot referenced by the CSP, the scheduler 10 first servicesthe VC waiting on or waiting in this slot. Scheduler 10 then schedules anew target time slot for this VC.

As can be seen from the above description, the functions of thescheduler and transmitter (for transmitting ATM cells) are essentiallycombined in the present embodiment. Every time a cell is sent on theoutput, the current value of the CCTP is used to “time stamp” thecorresponding VC. The difference between the current value of the CCTPand the previous value of the CCTP at which this VC was last serviced,equals the cell-to-cell spacing measured in terms of the link cell rate.This parameter called the cell to cell spacing (C2CS) 24 is part of theVC descriptor 20 and is used by the scheduler 10 to achieve a given cellrate for each VC. As an example, if the link cell rate is LR cells/secand the required cell rate for a VC is RCR cells/sec then thecell-to-cell spacing required is C2CS=LR/RCR.

When the scheduler 10 calculates a new target time slot, it services thequeues in a particular order of priority. First, the scheduler servicesVCs in low latency queue 17. If pending queue 17 is empty the schedulernext proceeds to service the VCs in the Current Slot High PriorityPointer (CSHPP), described below. Third, the scheduler services pendingqueue 16 for high priority traffic. If there is no high priority trafficthe scheduler next services the low priority pending traffic. Lastly, ifnone of the previous queues were serviced and the scheduler is in thenormal state, the scheduler then services the VCs in the Current SlotLow Priority Pointer (CSLPP) queue.

Referring to FIG. 2, there is shown a time slot ring 100 according to apreferred embodiment of the present invention scheduling mechanism. Inthe embodiment of FIG. 2, the time slots within the ring 100 are groupedinto clusters 102, wherein the ring includes cluster 0 through clusterN. Each cluster nominally holds a number of slots indicative of aCluster-Size 104. The time slot ring 100 of the instant embodimentcontains VCIDs which point to a first VC to be serviced in the cluster.The remaining VCs in a cluster are linked to the first VC via thepointers. In order to accommodate different classes of service, eachentry in the ring holds two specific VCIDs: a High Priority Pointer(HPP) 106 which services the VBR/CBR traffic classes and a Low PriorityPointer (LPP) 108 which services VCs belonging to the ABR/UBR classes.Assuming an arbitrary cluster size of 32, for example, the ring size cannow be reduced in size by a factor of 32.

In calculating a target slot, the scheduler 110 used in conjunction withthe time slot ring 100 calculates a target cluster, rather than theexact target slot. A Current Cell Time Pointer (CCTP) 112 incrementsevery cell time and points to the cell position which will be sent ifthe scheduler is not backlogged. In a similar fashion, a Current SlotHigh Priority Pointer (CS HPP) 114 and a Current Slot Low PriorityPointer (CSLPP) 155 point to the current high priority and currently lowpriority slots, respectively, which are being serviced.

In describing the operation of the scheduling mechanism 110 shown inFIG. 2, it will be understood that the scheduler can be in one of thetwo states: backlogged or normal. In the normal state, for everyincrement of the CSHPP or CSLPP 114, the CCTP 112 increments bycluster-size, since a number of cells equal to cluster-size have beensent. Accordingly, the normal state is defined as the case when CSHPPand the CCTP/Cluster-Size value will be in step or in synch with eachother. The CSLPP is not considered in determining the state of thescheduler. The scheduler 110 will then first service the high priorityqueue followed by the low priority queue. The low priority ring isserviced only when the high priority ring is in the normal state and thescheduler encounters an empty slot on the primary ring. Unassigned cellsare sent only if both the rings are in the normal state and the totalnumber of cells in both the queues combined is less than the clustersize. The remainder is a variable in the VC descriptor 20 and indicatesthe accumulated difference between the required cell-to-cell spacing andthe actual spacing.

The backlogged case is when the CCTP/Cluster-Size value is greater thanthe CSHPP. In this case, scheduler 110 will service only the highpriority queue. If a cluster 102 is empty, an unassigned cell will beemitted and the CCTP incremented before proceeding to the next cluster.This avoids the empty cluster look ahead requirement, but the trade offis that the scheduler 110, in effect, wastes or does not use one celltime for every empty cluster while it is backlogged. In connectiontherewith, the fraction of bandwidth not used is then at most(1/Cluster-Size), as will be explained.

Assume that the ring 100 has a capacity to hold R cells. The number ofclusters in the ring is therefore (R/Cluster-Size). A worst case for thering will be when the scheduler has to check all the clusters of thering and wastes one cell time for every cluster. Accordingly, thefraction of cells wasted is (R/(Cluster-Size*R), which is equivalent to1/Cluster-Size, as presented above.

Assuming a cluster size of 32, for example, the scheduler 110 will waste1/32 of the total bandwidth or about 3%. If the higher level calladmission control can ensure that no more than 97% of the bandwidth isallotted, the scheduler is then guaranteed to recover from thebacklogged condition within one iteration through the ring.

When a target slot is calculated, in the embodiment of FIG. 2, if thetarget slot turns out to be the current slot itself, then the targetslot can only be put in the slot subsequent to the current slot. Thisholds true for all the virtual channels whose cell to cell spacing isless than the cluster size. This problem is solved by defining anothervariable called burst-size. In the above example, every time the currentcell comes up for transmission, a burst of cells is sent. The C2CS isthen adjusted so that the effective rate equals the required rate. Asimilar problem is encountered when the remainder becomes negative andexceeds the C2CS. The scheduler can correct this by temporarily burstingone extra cell, and adding an amount equal to C2CS to the remaindervariable.

The scheduling scheme presented in connection with FIG. 2 has a strictupper limit on the number of accesses to the control memory per cell. Ascan be seen, however, the total memory required for the ring is nowsubstantially reduced. Considering the design for a 622 Mbps link,assuming a cluster size of 32 cells, the ring size required is only8294/32˜256. Assuming two bytes for the VCID, and two such pointers forhigh and low priority, we arrive at 1024 bytes. Such a memoryrequirement can easily be met by the internal cache memory of the SARchip. Since most of the accesses of the scheduler are to the ring 100, asubstantial reduction in the number of external accesses to the controlmemory is achieved.

As would be understood, the ABR VC can keep hopping between differentrates depending on the congestion control algorithm. The rate will bemodified by a re-assembly engine when it gets back the ResourceManagement (“RM”) cells which inform the scheduler to either reduce orincrease the rate. The re-assembly engine then calculates the requiredinter-cell gap, in terms of the number of time slots, and fills up theC2CS variable in the per VC record. The scheduler then automaticallyadapts to the new rate.

In the preferred embodiment described in connection with FIG. 2, the ABRtraffic which is on the low priority queue is serviced only after thehigh priority queue is serviced. In the event the scheduler is backlogged, the low priority queue is not serviced. Since achieving an exactbit rate is not critical for the ABR channels, the remainder mechanismcan possibly be turned off for these VCs.

As would be understood, the idea of having two queues: one for lowpriority and one for high priority can be extended to have a greaternumber of queues to provide for a larger number of traffic classes. Thiswould only be possible, however, at the expense of increased memoryrequirements.

Referring to FIG. 3, another preferred embodiment of the presentinvention splits adds to the one high latency time slot ring asdescribed above, a second high latency time slot ring, each ring usedfor a different level of priority of VCs. One of the rings, fast ring30, will include small cluster sizes comprised of M small slots 31 forhigh speed, high priority VCs. The other ring, slow ring 40, will havelarger cluster sizes comprised of N larger slots 41 for slow speed,lower priority VCs. There are two advantages to this new embodiment.First, higher speed VCs can be given higher priority and consequentlysuffer less jitter compared with the low speed VCs. Second, the overallmemory requirement are lower compared with using a single ring.

The scheduler when calculating the target slot will first see if it canbe inserted in fast ring 30, to the extent they fit and to the extentthat C2CS of the VC is less than the remaining space in the high speedring 31. If the target slot can not be fitted on the fast ring 30 itwill be put in the appropriate slot on the slow ring 40. The slow andfast rings have their respective current slot pointers and also willhave a state either Normal or backed-up. The target slot is calculatedwith respect to the CCTP as translated to either of the rings. Thescheduler, after servicing the low latency pending queue as describedabove, will first service the fast ring 30 and the slow ring 40 will beserviced only if the fast ring 30 is in the normal state. The minimumnumber of clusters required in fast ring 30 equals R, where R is theratio of the size of the cluster in slow ring 40 to the size of thecluster in the fast ring 30. However, better performance is realizedwhen the fast ring is made to have a larger number of clusters than theminimum R. As an example, consider an implementation where the clustersin slow ring 40 is 256 slots and that of fast ring 30 is 32 slots. Theminimum number of clusters required in fast ring 30 is 256/32=8. Thesize of slow ring 40 is dependant as before on the slowest speed to besupported on any given VC.

In considering latency with respect to the present invention schedulingmechanism, it is known that some applications such as multiprocessinguse a local ATM network to interconnect a group of work stations. Theseapplications require VCs with very low latency. There are three mainsources which contribute to the latency end-to-end. First, latency dueto the protocol stack at the end point work stations. This has to beminimized by bypassing the regular stack for these low latency VCs.Second, latency in the network interface card, which includes thescheduling delay and the delay due to queuing of the cells afterscheduling to increase the throughput. Third, the latency suffered inthe ATM switching network.

The first source of latency within a network interface card is due tothe scheduling delay. The solution to this problem is to create aseparate queue reserved for the low latency VCs. Thus, for every celltime, the scheduler should first look at the low latency queue beforeservicing any other queue. Accordingly, the low latency queue has thehighest priority. Also while the scheduler is servicing the low latencyqueue, the CCTP keeps incrementing. The remainder mechanism will ensurethat the transmit rate of the VCs on the high priority queue will notsuffer as long as the low latency traffic is kept small in comparison tothe other traffic. The other possible source of latency is the queue ofcells after the scheduler. A small queue of cells is usually kept to“smooth” the traffic and take care of possible delays in scheduling dueto overloading of the control memory bus or the host bus. For thepresent invention, this queue should be kept to the absolute minimumsize.

From the above, it should be understood that the embodiments described,in regard to the drawings, are merely exemplary and that a personskilled in the art may make variations and modifications to the shownembodiments without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An apparatus for scheduling communicationstraffic from a node over an ATM network such that a low latencyrequirement of certain virtual channels (VCs) can be satisfied whilemaintaining overall high throughput, said apparatus comprising: a VCtable including an array of VC descriptors, each said VC descriptorassociated with a virtual channel identifier (VCID) pointing to said VCdescriptor; at least one time slot ring including an array of timeslots, wherein each of said time slots contains a VCID of a VC to beserviced; processing means for placing VCIDs in said time slots of saidat least one time slot ring, wherein said time slots are circularlyprocessed and said placed VCIDs are retrieved from saidcircularly-processed time slots in a continuous fashion to scheduletransmission of associated ATM cells in said network; and a plurality ofpending queues, said plurality of pending queues including a firstpending queue for storing VCIDs associated with low latency VCs, whereinsaid VCIDs associated with low latency VCs are serviced from the firstpending queue before servicing VCIDs other than said VCIDs associatedsaid low latency VCs.
 2. The apparatus of claim 1, wherein said timeslots are grouped into a plurality of clusters, each of said clustersincluding a predetermined number of time slots equal to a cluster size,wherein each of said clusters is associated with at least first andsecond pointers of differing priority for accommodating transmissionunder various classes of service from said node, wherein each of the atleast first and second pointers in a cluster identifies a VCIDassociated with a current VC to be serviced, and the identified VCIDpoints to a VC descriptor that either identifies a next VC to beserviced or indicates that the current VC to be serviced is the last VCof associated priority to be serviced in the cluster.
 3. The apparatusof claim 2, further including a current cell time pointer (CCTP), acurrent slot high priority pointer (CSHPP) and a current slot lowpriority pointer (CSLPP), wherein only VCs identified by the CSHPP areserviced during a backlog condition in said time slot ring, said backlogcondition existing when a CCTP/cluster-size value exceeds a CSHPP value.4. The apparatus of claim 3, wherein if a current cluster is empty, saidprocessing means schedules a transmission of an unassigned cell beforeproceeding to a next cluster.
 5. The apparatus of claim 4, furtherincluding a burst_size variable, wherein each time said processing meansschedules a target slot for a currently active slot, a burst of cellsaccording to said burst_size variable is transmitted during saidcurrently active slot.
 6. The apparatus of claim 3, wherein said CSHPPand said CSLPP are operable to point to current high priority andcurrent low priority time slots, respectively, which are currently beingserviced.
 7. The apparatus of claim 1, wherein said at least one timeslot ring is included in internal cache of a segmentation andre-assembly (SAR) applications specific integrated circuit (ASIC). 8.The apparatus of claim 1, wherein said at least one time slot ringincludes two time slot rings, each of said time slot rings correspondingto a different cell-to-cell spacing (C2CS) requirement, each said C2CSrequirement consistent with quality of service (QoS) requirements forVCs to be serviced by the corresponding time slot ring.
 9. A method forscheduling communications traffic over an output link in an ATM networksuch that a low latency requirement of certain virtual channels (VCs)can be satisfied while maintaining overall high throughput, said methodcomprising the steps of: providing at least one time slot ring includingan array of time slots; placing at least one virtual channel identifier(VCID) of a VC to be serviced in a time slot; referencing a VC tableincluding an array of VC descriptors, wherein each said VCID placed insaid time slot points to a VC descriptor in said VC table, said VCdescriptor including pertinent information regarding processing of aspecific VC; circularly processing said time slots of said at least onetime slot ring in a continuous fashion at a predetermined rate to enablescheduled transmission of ATM cells associated with said at least oneVCID to be serviced over said output link; queuing VCIDs associated withpending low latency VCs in a first pending queue; queuing VCIDsassociated with pending VCs other than said pending low latency VCs in asecond pending queue; servicing the first pending queue before servicingthe second pending queue so that said pending low latency VCs arescheduled for transmission before other pending VCs; and placing VCIDsfrom the second pending queue into time slots in the at least one timeslot ring for subsequent servicing.
 10. The method of claim 9, furthercomprising the steps of: dividing said at least one time slot ring intoa plurality of clusters, each of said clusters including a predeterminednumber of time slots equal to a cluster-size; and associating each ofsaid clusters with a set of pointers, one cluster per set, each setincluding at least first and second pointers of differing priority foraccommodating transmission of ATM cells under various classes of servicefrom a node, wherein each of the at least first and second pointers in acluster identifies a VCID associated with a current VC to be serviced,and the identified VCID points to a VC descriptor that either identifiesa next VC to be serviced or indicates that the current VC to be servicedis the last VC of associated priority to be serviced in the cluster. 11.The method of claim 10 further comprising the steps of: furnishing toeach said at least one time slot ring a current cell time pointer(CCTP), a current slot low priority pointer (CSLPP) pointing to acurrent low priority time slot being serviced, and a current slot highpriority pointer (CSHPP) pointing to a current high priority slot beingserviced; identifying backlog conditions in said time slot ring, saidbacklog conditions existing when a CCTP/cluster-size value exceeds aCSHPP value; and servicing only VCs identified by the CSHPP duringexistence of the backlog conditions.
 12. The method of claim 11,including the step of scheduling a transmission of an unassigned cellbefore servicing time slots of a next cluster if a current cluster isempty.
 13. The method of claim 9, wherein each of said VC descriptors insaid VC table includes a cell-to-cell spacing parameter for anassociated VC, wherein said cell-to-cell spacing parameter is defined asa transmission rate of said output link divided by a required cell ratefor said associated VC, further including the step of altering a cellrate for said associated VC in said time slot ring by altering saidcell-to-cell spacing parameter for said associated VC.
 14. The method ofclaim 13, further including the steps of: calculating remaindervariables for inclusion in each of said VC descriptors, one remaindervariable per VC descriptor, each said remainder variable defined asaccumulated difference between required cell-to-cell spacing and actualcell-to-cell spacing for a VC corresponding to a VC descriptor withinwhich each said remainder variable is included; and bursting apredetermined number of extra ATM cells associated with VCs withnegative remainder variables.
 15. The method of claim 11 wherein saidcircularly processing step comprises the additional steps of firstprocessing time slots associated with a current cluster in a first oneof said at least one time slot rings, and so long as a backlog conditiondoes not exist, then processing time slots associated with a currentcluster in a second one of said at least one time slot rings.